This project investigates the molecular pathways that govern the programmed death of T lineage cells in the mature peripheral immune system. Five areas have been addressed: 1) The role of co-stimulatory and co-receptor interactions in modulating a T cell receptor-induced death signal, 2) The involvement of Fas and TNF in mature T lymphocyte death, 3) The relative importance of p53, Bcl-2, Bcl-X and other regulators of apoptosis in mature T lymphocytes, 4) Identification of downstream mediators of Fas and TNF-induced apoptosis, and 5) The molecular pathway that leads to lymphokine withdrawal apoptosis in T cells. We compared the signaling requirements for activation to those required for T cell receptor (TCR)-driven programmed cell death (PCD). We found that ligation of the CD4 co-receptor but not the CD28 co-stimulatory molecule strongly influenced TCR-induced PCD. We have found that apoptosis in mature CD4 T cells can result from interactions between Fas (Apo-1/CD95) and Fas ligand whereas that of mature CD8 T cells is due to tumor necrosis factor (TNF). Analyses of mice with homozygous null mutations in the p55 or p75 TNF receptor genes show that the p75 receptor is sufficient for T cell apoptosis. We have found that the active death pathway resulting from TCR induction of Fas ligand and TNF is not influenced by p53, Bcl-2, or Bcl-X which have a role in regulating lymphocyte apoptosis in other contexts. We have shown that engagement of Fas or the TNF receptor results in the activation of a special set of cysteine proteases, termed caspases, that carry out specific proteolytic events leading to programmed cell death. We have identified a novel cellular substrate for these enzymes that is cleaved during Fas-induced death of non-transformed T lymphocytes. Most recently we have found tyrosine phosphorylation events that are associated with lymphokine withdrawal apoptosis in T cells. We are investigating the functional importance and molecular triggers of these signalling events to understand this form of passive lymphocyte death that may play an important role in T cell homeostasis. Together these studies will provide important insights into the molecular pathways that control T cell apoptosis which plays a vital role in regulating T cell-dependent immune responses.